Water-rock reactions during gamma-ray irradiation

The effects of gamma-ray irradiation on water-rock reactions are investigated by comparing leaching experiments with and without irradiation. Basaltic rock fragments were leached for 887 h with distilled, deionized water, while moderately flushing with air. During the leaching a total of 10 aliquots were removed and analysed by ion chromatography and instrumental neutron activation analysis (INAA). The leached amounts of Na, K, Ca, Cs, Mg, Co, Ba, Zn and Sc were higher for the irradiated run than those for non-irradiated, but the leaching of W was depressed by irradiation. The differences in the amount of the dissolved elements between irradiated and non-irradiated runs for the same time (radiation release) correlate with the absorbed dose of water. Four temporal variation types of the radiation release are recognized. The radiation releases of Na, K, Ca and Cs increased linearly with irradiation time and the rate increases are on the order of 10(-9) to 10(-10)/rad(H2O); those of Mg, Co, Ba and Zn approached constant values. The radiation release of Sc increased linearly after a threshold value is reached, and that of W decreased linearly. These results show that water-rock interaction is promoted by gamma-ray irradiation in most cases. Of the major basalt constituents, the elements that show a linear correlation of the radiation release with the absorbed dose are contained more in the feldspars, while the elements that approach constant values with increase of absorbed dose are contained more in the mafic minerals. It is conceivable that during irradiation the behavior of an element is controlled by the reaction kinetics between water and each mineral species. Water radiolysis products generated by irradiation are likely to be the main cause of increased dissolution; they also cause changes in the valences of metal-ions in aquatic systems. The valence changes are likely a key to understanding the reaction kinetics during gamma-ray irradiation. Copyright (C) 1996 Published by Elsevier Science Ltd.